(Not Applicable)
1. Field of the Invention
This invention generally relates to gas cleaning systems, and more specifically to a biologically-based absorbing apparatus and method to reduce emissions from fossil burning units.
2. Description of the Related Art
The U.S. produces an estimated 1.7 billion tons of CO2 annually from the combustion of fossil fuels. CO2 is a reflector of infrared radiation, so its presence helps xe2x80x9ckeepxe2x80x9d heat in the atmosphere, making the surface temperature warmer than if there was no CO2 in the atmosphere. It is estimated that at present growth rates, CO2 levels in the atmosphere will increase from 350 ppmv (at present) to 750 ppmv in as little as 80 years. In fact, to level CO2 concentrations at 550 ppmv, we will have to reduce net CO2 emissions by over 60% from 1990 levels during the next 100 years.
Even if an expensive option for CO2 removal is discovered, which is by no means a certainty, CO2 xe2x80x9cdisposalxe2x80x9d is problematic. U.S. industries consume only 40 million tons of CO2, produced at a much lower price than possible by removing CO2 from flue gas. Therefore, increased consumption of CO2 appears limited, and options for expanded use appear limited and costly.
Sequestration of CO2 in large bodies of water or in deep mines appears to be the most viable present option. However, sending CO2 into the ocean or an abandoned mine is a limited solution. There is no known exact time scale for storage of CO2; it may be centuries, but it also may only be decades. At best, these are temporary solutions. Further, the transportation issues are considerable, even for the less than 30% of all U.S. fossil-fuel burning power plants that are within 100 miles of an ocean. Existing power plants, with capital values in the hundreds of billions of dollars, are at risk if tens of thousands of miles of specialized pipelines must be installed to transport separated CO2.
The use of ocean-based sinks could present significant problems. It will be necessary to add large amounts of iron to the ocean to use the vast quantities of CO2 stored in the sinks, resulting in uncontrolled growth of certain organisms. Weed plankton, the most likely organisms to grow, will not provide sufficient nutrients for the food webs, and there is a high probability of significant negative environmental impact. In the case of CO2 stored at the bottom of the ocean in lakes, the adverse effects on the ocean-floor ecosystem cannot be predicted, but are likely to be considerable.
Another existing option involves biological carbon sequestration in outdoor ponds. However, there are inherent inefficiencies related to this solution for CO2 sequestration, primarily due to the amount of cyanobacteria that can be grown in a given volume. For example, if 2,000,000 m2 of photosynthetic surface area is required for 25% reduction of CO2 emissions from a power plant, that is equivalent to almost 500 acres of surface. Very few existing plants have 500 acres available to them and fewer could afford to convert 500 acres to a shallow lake or raceway cultivator. Also, there are serious questions about how to distribute the flue gas (or separated CO2) into the lake for maximum growth, not to mention what to do with the gas once it bubbles to the surface. The flue gas would have to be collected again and redirected up a stack to meet other emission requirements. Further, maintaining such a large xe2x80x9clakexe2x80x9d during a Midwestern winter would be problematic.
Clearly, other approaches for CO2 control are needed. Research to develop a robust portfolio of carbon management options, including safe and effective photosynthetic carbon recycling, will enable continued use of coal in electrical power generation. Despite the large body of research in this area, virtually no work has been done to create a practical system for greenhouse gas control, one that could be used with both new and existing fossil units.
A method for removing a carbon-containing compound from a flowing gas stream is performed by interposing in the stream a membrane having photosynthetic microbes, such as algae and cyanobacteria, deposited thereon. Applying water and nutrients to the membrane sustains the growth of the microbes, and increasing the volume of water harvests the microbes from the membrane.
The invention also contemplates an apparatus for removing a carbon-containing compound from a flowing gas stream has a membrane interposed in the stream. The membrane has photosynthetic microbes, such as algae and cyanobacteria, deposited thereon.